Hopefully this will indeed become an in-depth discussion worthy of the Saga forum. Because frankly I don't think this issue gets enough attention.
TPM and AOTC get bashed for poor acting, and sometimes I wonder if people haven't forgotten that SW acting's never been that great.
So I sat down and thought about it... and here's what I've come up with. Just my opinion, mind you.

SKYWALKER HERO: Anakin vs Luke
PREQUELS: Jake Lloyd did his best but was often pretty darn bad. Hayden Christensen delivers a textured portrayal of an angst-ridden young Vader, and doesn't forget to take in history -- he nails a James-Earl-Jones-ish speaking cadence, Dave Prowse's walk, Mark Hamill's fighting stances. To me, this guy just IS Anakin.
CLASSIC: Mark Hamill was all right as the whiny farm boy, but just couldn't keep it up in ESB or ROTJ. When a guy can't hold his own acting against a green Muppet, something is wrong.
ADVANTAGE: Prequels by a slim margin

DASHING OLDER COMPANION: Obi vs Han
PREQUELS: Ewan McGregor didn't get enough opportunities to strust his stuff in TPM, but still made an impressive mark with his impression of Alec Guinness. With AOTC, he got the spotlight and star billing, showing us things we never knew about this character while retaining the charm and reserved, very British dignity of Sir Alec.
CLASSIC: Harrison Ford plays Harrison Ford. (Just as Woody Allen plays the same part in every movie... just as Rick Moranis always plays the nerd... just as Jeff Goldblum plays Jeff Goldblum.) Admittedly this was before he was just playing Harrison Ford in every movie, but you can tell he's just a little tired of the Harrison Ford act by ROTJ.
ADVANTAGE: Prequels

TOKEN FEMALE: Padme vs Leia
PREQUELS: Natalie was the worst cast member in all of TPM in my opinion. She rose to passable-but-nothing-special in AOTC. She does not seem comfortable with the '30s-style dialogue SW demands.
CLASSIC: Carrie Fisher. Beautiful, brave, bold, the one and only classic space princess. She had more spunk in her pinky than Natalie showed in all of TPM.
ADVANTAGE: Classic, big time

WISE OLDER JEDI: Qui-Gon/Mace vs Obi
PREQUELS: Liam Neeson gave one of the best performances in SW history, giving Qui-Gon depth and presence. Then Qui-Gon had to go get himself killed. While Samuel L. Jackson is a passable Mace, I'm not really sure what he's doing with the character. Sure, he fights well and looks imposing/interesting, but couldn't there be more?
CLASSIC: Sir Alec Guinness lent that crazy little sci-fi movie the depth and dignity it needed. He will be missed.
ADVANTAGE: Classic

I had to sort of fudge the bad guys here because I didn't want to bring Ian McDiarmid into this 'cause he's (excellent, I might add) in both PT and OT movies. Bear with me.

SITHLY BAD GUY: Dooku vs Vader
PREQUELS: Christopher Lee's the man, don't get me wrong. But he doesn't get enough of a chance to strut his stuff.
CLASSIC: A guy in a suit and a voice-over. I feel bad making this choice, because there was probably more acting work involved in bringing Dooku to life. But Vader's an icon. Sorry, Christopher...
ADVANTAGE: Classic

NEW GUY INTRODUCED IN SECOND EPISODE: Jango vs Lando
PREQUELS: Temuera Morrison got just one scene to really make an impact, and he made good.
CLASSIC: The Colt 45 guy?
ADVANTAGE: Prequels

Someone actually gave props to Ewan's Obi-Wan over Han, Jar Jar over Chewbacca, and the Disposable Silver Rocketeer over the smooth and silky Billy Dee?

Someone obviously hasn't got their head on straight. The Chewie thing alone proves that those choices are psychotic. And Jabba's fat-filled tail flicker had more personality than that bucketheaded buffoon Fett.

There have been some amazingly talented actors in both the PT and the OT. The only person I've really been at all disappointed in has been Natalie Portman, because I KNOW the girl can act. I'm not saying her performances have been bad; she really had some great moments as Queen Amidala in Episode I, and when she says "No, I'd call it aggressive negotiations," in Episode II, you see a spark of that feistiness we all loved about Princess Leia.

Of course, Padme and Leia are different characters, and Padme is supposed to be a reserved and level-headed politician. I'm sure Nat's only doing what King George tells her to, but at times she does come off as a bit boring.

I do expect her to "redeem" herself in Episode III and show a bit more emotion; if she doesn't, I'll really be disappointed. She was wonderful in the meadow picnic scene in AotC, and also in the love pledge scene, so I hope we can see more of that side of Natalie in the next one.

As for people always bashing Jake Lloyd--first of all, he was what, eight when he started filming Episode I? There were times when he overacted a bit, but all in all, he was what he was...a young boy. I thought he did pretty well considering.

As I look at this issue, I would have to agree with J-Solo. The director has the most impact on how an actor/actress portrays a character on screen. Sure, the actors themselves are what give the character personality and emotions, but it's the director that has the final say. And in my opinion, it shows between the OT and PT. The OT had better acting over the PT mostly because of the change in directors, IMHO. Now, this isn't a bad thing and I'm not bashing GL for his directing skills. GL had a tendancy not to direct the actors enough on what to do and how to act and, as a result, got bad to okay portrayals of his characters.

So you would say ANH's acting is on par with the PT's? Lucas directed that too, you know. He also wrote the screenplay.

I see what you mean, but there's a difference. When ANH was filmed, George Lucas WAS a director, and a good one, I would add. He had directed THX 1138 and American Graffitti, two great films.

Lucas "retired" as a director after A New Hope. But, I don't know why, he decided to come back to the chair for the prequels. And after all of these years he just lost his touch, I think. But I'd say that the acting in ESB is better than ANH. Jedi too. I think it's because of the directors.

I've made fun of the acting in the OT, but I accept the OT, because I'm used to it. Maybe when twenty years pass, I will come to accept the acting in the prequels.

I don't like the way lines were delivered in TPM and AOTC. I like Natalie Portman, but she just doesn't shine in SW. I talked to my sister about how cool Leia was, and that Padme just seems so lame. That if Carrie Fisher were to deliver the line, it would sound this way, rather than that way.

I think Fisher had a timeless classic quality about her acting, while Portman tends to lean towards acting that screams modern-day. And what I mean by modern-day, is that she acts like the character lives on earth in the 21st century. And I fear that it will not stand the test of time.

I do not think one is better then the other. I think both have their own style. PT is in a different time then the OT. So the acting is going to be different. Hayden I think gave the best emotional scene yet with the confession seen. Felt like he was going to go Vader on Padme there. I think ESB had the better acting in that series. You just wanted to slap Luke for being impatient with the training. And Han was a smoothie. So I can not really say one is better then the other. Both have some great acting in them. That is why I enjoy all five films. I can take somthing from each film to enjoy.

It's hard to analyse the acting in isolation. I really think it has a lot to do with casting, for instance Ralph Feinnes is an excellent actor but put him in The Avengers and he stinks, this is 'cos he hasn't got that easy charm which is required.

Another factor is the script and the direction, we know that Portman and McGregor can act, we've seen them give good performances in other movies, but in TPM they're dull. Why? well they just seemed to be reciting their lines with little connection to anything, no chemistry.

Jake Lloyd's performance was mostly bad, with child actors it really is down to the director to cast the right kid and really work closely with them to get it right.
JJ had a lot of work and effort put into his performance, unfortuantely it was an embarassment .

The acting in the OT is much better, they are perfectly cast, they have great dialogue, plenty of chemistry and a script which cares about those characters.

I think that Billy Dee Williams does not get a fair shake for his portrayal of Lando in the OT, as his character might be the most underrated figure in the entire OT. He inflects a great personality into a character who really could have turned out poorly given the way he quickly went from a traitor to an almost instantly accepted good guy in ROTJ. IMO, Lando's character is far better and more important than a bounty hunter who is popular mostly because of his cool, gadget-filled suit. Don't get me wrong, I still definitely like Jango (and Boba, for that matter) but really, I often wonder how many people would even care one iota about the Fetts if it weren't for their costumes and mechanized voices.

I agree with stacysatrip, the only person I could fairly say I was disappointed with would be Natalie Portman.

Having seen her in other films I know she's a very talented actress...and having seen Lucas get very good performances out of some of his actors, it can't be him to blame either (some will disagree, natch).

I wonder if it's because Padme is a tough character for her to play - Padme's been a diplomat all her life, someone's that very straight-laced usually and one of these maddeningly calm 'keep my head in a crisis' type of people. She keeps her emotions very much in check.

A lot of Padme's character seems against Portman's natural personality, and it always helps when the actor in question has some kind of link with the part they are playing. Only the very best of actors, maybe a dozen or so over the last century of cinema, can convincingly play 'against type'.

I've made fun of the acting in the OT, but I accept the OT, because I'm used to it. Maybe when twenty years pass, I will come to accept the acting in the prequels.

It's a very good point IMO.

To me, the acting in the OT is plain and simple, just as the main characters. Maybe this is one important reason why most people can appreciate the OT---because it's apparent, thus easy to understand. The PT, on the contrary, is subtle and complex.

Take Natalie Portman and Carrie Fisher for example. I feel that Natalie's facial expression is much subtler than Carrie, but Carrie's voice is more attractive, she delivered her lines better.

I prefer subtlety, so I think Natalie is better than Carrie, and the acting in PT is better than OT.